A process for optically resolving DL-phenylalanine with the aid of optically active mandelic acid is already known [see Nippon Kagaku Zasshi 92(11), 999-1002 (1971)]. However, when the inventors of the present invention made a follow-up of the well-known process, it was found that the process was extremely disadvantageous for industrial practice because the solubility of the phenylalanine.mandelic acid complex in water was extremely low (2.6% in terms of phenylalanine), the optical purity of the resolved optically active phenylalanine was as high as about 80% unsatisfactorily, even after considerations of various conditions and, because of the low initial concentration as mentioned above, the ratio of the obtained optically active phenylalanine.mandelic acid complex to the mother liquor of resolution was extremely low.